This invention relates to an electron gun structure having a plurality of electron lenses along the electron beam path.
A color cathode ray tube is provided with an electron gun for generating three electron beams which excite three respective color phosphors on a screen. A well known electron gun structure is an in-line type which generates three parallel electron beams within a plane. Each of the electron gun units of this structure is contained within a united body. FIG. 1 of the accompanying drawing shows one example of an in-line type gun widely used in color cathode ray tubes. The electron gun structure 10 includes three electron units 11, each comprising a cathode 2 including filament 1, a first grid electrode 3, a second grid electrode 4, a third grid electrode 5, and a fourth grid electrode 6. All of the grid electrodes are common to the gun units, for example, the first grid electrode 3 is common to gun units 11. A main electrostatic lens 7 is formed, respectively, in each electrode gun unit 11 between the third grid electrode 5 and the fourth grid electrode 6. The third grid electrode 5 is a can-type electrode consisting of two cup-shaped metal members joined together at their open ends. Each surface of electrode 5 comprises three openings; a center electron gun axis Z.sub.G is aligned with the center opening, while side electron gun axes Z.sub.B and Z.sub.R are aligned with the respective side openings. Reference numerals 5.sub.B, 5.sub.R and 5.sub.G designate the three openings of electrode 5 which face fourth grid electrode 6. Grid electrode 6 is made of a cup-shaped metal member having three axial openings 6.sub.B, 6.sub.G, and 6.sub.R. The center of opening 6.sub.G coincides with the center electron gun axis Z.sub.G, while the center of both side openings 6.sub.B and 6.sub.R are offset from the respective side electron gun axes Z.sub.B and Z.sub.R.
As well known, the three electron beams generated must converge for proper color purity. In some prior art devices, this convergence may be obtained by inwardly tilting both side electron gun axes. However, tilting of the side electron gun axes results in imprecision of the gun assembly. Consequently, the structure of FIG. 1 relies upon another prior art method for convergence. Namely, the center electron beam 8 follows a straight path, coincident with the electron gun axis Z.sub.G through opening 5.sub.G of the third grid electrode and opening 6.sub.G of the fourth grid electrode. On the other hand, each side electron beam 8.sub.B and 8.sub.R will deflect or bend inwardly toward the center of the screen since the respective center of openings 6.sub.B and 6.sub.R are offset from the electron gun axes Z.sub.B and Z.sub.R, respectively. For example, the amount of offset utilized is about 0.15 mm. When one grid electrode is offset or disposed at an angle to the other grid electrode, the electron field formed therebetween is asymmetrical. An electron beam passing through such asymmetrical field will bend. The amount of deflection of the electron beam resulting from the offset or angle design depends critically on the amount of offset or angle, respectively, and the potential difference between the grid electrodes.
The amount or angle .theta. of deflection of the electron by an asymmetrical electron lens is expressed by equation (1), where k is constant, p is the amount of offset normalized by the electrostatic lens diameter, and q is a potential ratio of the electrostatic lens. EQU .theta.=(k)(p)(q) (1)
Since the amount of deflection is proportional to q, if the potential across grid electrodes q is not optimum, the electron beams will diverge rather than converge. Moreover, varying the grid voltage to set the optimum value for convergence will also affect focusing. In fact, setting the grid voltage for proper convergence will not necessarily produce proper focusing and vice versa. Consequently, the prior art has required the adjustment of the permanent magnets, attached to the outside of the tube, to partly aid in optimizing both convergence and focusing. For example, pre-set-type color cathode ray tubes have been developed in recent years in which the convergence and focus are set before the tube is attached to a TV set. Frequently, when the tube is attached to the TV set, the electron gun operating voltage is not correct for proper focusing and convengence, and readjustment is necessary. Such readjustment of the tube requires varying both the grid voltage and the position of the permanent magnets; these adjustments are very critical and time consuming.
Another prior art color cathode ray tube comprises an electron gun surrounded by a permanent magnet member within the tube. The magnet member is magnetized from the outside of the tube to converge three electron beams on the center of the screen. Readjustment of this structure, however, is virtually impossible after the tube is attached to the TV set.
Therefore, the electron guns of the prior art have the disadvantage of requiring critical and time consuming readjustment when the tube is attached to the TV set to provide proper focusing and convergence.